The relatively wide bandwidth of light that may be transmitted through a conventional optical fiber enables multiple light signals, each at a different wavelength, to be multiplexed and transmitted simultaneously over the same optical fiber. Such a technique is called wavelength division multiplexing (WDM). It is common for a single optical fiber to simultaneously transmit 16 or more multiplexed channels for any form of communication, including telephone communications and cable television.
In WDM, the signals (either electrical or optical) to be conveyed on each channel are converted into light signals within a narrow band of wavelengths (e.g., 2 nanometers) associated with a particular channel. A 16 channel WDM would use a total bandwidth of about 32 nanometers. A common center wavelength is on the order of 1500-1600 nanometers.
Converting an electrical or optical signal into a particular narrow band of wavelengths is well known. For example, an electrical signal may be applied to a particular type of laser diode which generates wavelengths within a particular bandwidth. Other techniques may include converting the electrical signal into a light signal and eliminating unwanted wavelengths. Some devices for extracting a specific narrow band of wavelengths from an optical signal include: 1) a tuned waveguide; 2) a diffraction grading; 3) a taper filter; and 4) other types of filters, such as a coated silica substrate where certain wavelengths are refracted and other wavelengths are reflected.
The process of causing the optical signals to be within a particular narrow bandwidth also typically causes the optical intensities to differ for each channel. As a result, after the optical signals for the channels have been limited to their respective optical bandwidths, such as shown in FIG. 1, each of these optical signals must be attenuated so that the light intensity transmitted is equal for each channel and is of a predetermined level. This is so that the transmission performance for each channel is predictable. Such attenuators for each of the three channels (1, 2, and n) shown in FIG. 1 include attenuators 12, 13, and 14 for attenuating the optical signals in optical fibers 16, 17, and 18, respectively. Similar attenuators reside in a demultiplexer 19.
FIG. 2 illustrates the intensity levels of the optical signals in each of the three channels, each optical signal being within a different narrow bandwidth of light. As seen, the intensity of the optical signal in channel 1 prior to attenuation is greater than that of the optical signals in channels 2 and 3, and the optical signal in channel 3 is greater than the intensity of the optical signal in channel 2. Attenuators 12, 13, and 14 serve to equalize the intensity levels of the three channels by selectively lowering the overall intensity of the higher intensity signals to equal that of the lowest intensity signal. One such attenuator will be discussed later with respect to FIGS. 3, 4A, and 4B.
The light outputs from the attenuators 12-14 are then applied to optical fibers 20, 21, and 22 and combined into a single optical fiber 24 so as to multiplex the n channels onto a single optical fiber. Hence, the device of FIG. 1 acts as a multiplexer to simultaneously transmit multiple channels, each at a different light bandwidth, along the same optical fiber. Additional multiplexers may be employed to multiplex additional channels on other optical fibers. The optical fibers may then be bundled in a cable for transmitting many optical signals.
Ultimately, the signals on the optical fiber 24 are demultiplexed by a demultiplexer 19 to separate out the various wavelengths of light into separate channels using well known means. These separate channels are then attenuated to have equal, predetermined intensities and converted into electrical signals, if required, for various applications such as by using photodetectors. Such demultiplexers include detraction gratings and filters which may be tuned to transmit a narrow range of predetermined wavelengths.
The attenuation levels in the multiplexer and demultiplexer may be determined empirically.
One popular prior art technique for attenuating the intensity of a light output within a narrow band of wavelengths uses a neutral density filter for each of the wavelength bands of interest. Such a filter removes a selected amount of light depending on where the light impinges upon the filter. FIG. 3 illustrates a neutral density filter 30 composed of a silica substrate 32 with a coating 34 composed of material for progressively absorbing the light output of a fiber optic cable 36 as filter 30 is moved in the direction of arrow 38. The percentage of absorption of light output from cable 36 with respect to each area of filter 30 is identified in FIG. 3. The light exiting filter 30 is received by a fiber optic cable 40. It would be understood that additional optics, such as collimators, may be used at the ends of the fiber optic cables 36 and 40 to cause the light between the two cables to be collimated.
The filter 30 is adjusted in the direction of arrow 38 using a micrometer to select the desired amount of attenuation.
FIG. 4A illustrates the ideal light energy versus time for a number of pulses of the attenuated light received by fiber optic cable 40. In reality, however, this light signal contains ripples and other distortions, as shown in FIG. 4B, due to reflections at the interface of filter 30 causing constructive and destructive interference. Further, an inherent property of the silica 32 and the coating 34 is that there is always some attenuation even at the minimum attenuation level of filter 30.
What is needed is a light attenuator for a WDM system which is inexpensive, reliable, and does not suffer from the performance drawbacks of the prior art attenuators.